1. Field of the Invention
The present invention relates to a method for fabricating thin films of various materials by electric glow discharge in gas. It especially relates to a sputtering method having a high sputtering efficiency at a low gas pressure which assures low contamination and a high grade of sputtered film, while keeping the consumption of gas low.
Sputtered thin films are used in variety of industries, such as production of semiconductor devices, thin film circuit boards, optical equipment, etc. There are several methods for forming thin films. Among them, electrical sputtering is one of the most widely used methods and it can be used for sputtering various materials, such as metals, dielectrics or semiconductor materials.
2. Description of the Related Art
Sputtering is a method whereby a material to be sputtered is bombarded with ions in a plasma formed by electric discharge, and the material is sputtered onto an object in the form of a film. Recently, in order to increase the sputtering efficiency, a magnetic field has been applied to focus the ions on the target formed by the material to be sputtered. Such method is called in the art, magnetron sputtering.
The quality of the sputtered film depends largely on the sputtering conditions. Further, modern applications require high stability and purity of the thin film so produced. For example, recently, the design of very large scale integrated (VLSI) semiconductor circuits, and devices using such, have become very delicate. Therefore, the quality of the wiring, made of an aluminum thin film, affects the reliability of the devices. If oxygen or humidity are included in the sputtered aluminum film, the wiring often looks white because of small unevenness formed on the aluminum surface. The electrical conductivity of such wiring is lower than designed, and can cause disconnection by migration of aluminum.
The sputtering equipment must be very carefully maintained. The sputtering chamber should be degassed by baking it in vacuum before it is used, and the inert gas should be purified. Still, some impurities stray into the plasma, and cause contamination of the sputtered film. This decreases the reliability of the device and the production yield. Such contamination may be due to minute leakage in the sputtering chamber itself and/or to residual gas which is very difficult to perfectly remove from the chamber by preliminary degassing. Sometimes, this residual gas is brought into the sputtering chamber together with the object itself. For example, if an aluminum film is formed over a phosphosilicate glass (PSG) substrate, humidity is easily adsorbed by PSG and thus brought into the sputtering chamber. The adsorbed humidity is released by the sputtering, and may poison the sputtered aluminum film.
In ordinary sputtering apparatus, an ion plasma is formed by glow discharge in argon gas at a pressure of about 10.sup.-3 Torr. There is a range of desirable pressure for keeping a stable plasma in the sputtering chamber. Too high or low pressure, than the desirable pressure, does not maintain a stable electric discharge, and thus cannot maintain a stable plasma. In order to maintain such stable plasma, the pressure of gas in the sputtering chamber is controlled by injecting inert gas, argon for example, into the chamber or controlling the pumping speed of the evacuation pump to balance the pumping speed or the evacuation pump with the injected gas flow to the desired pressure.
In order to avoid the undesirable effect of any residual gas, higher pumping speed is better. But if the pumping speed is very high, the flow rate of the injection gas must be high to keep the pressure in the sputtering chamber at the desired pressure for a stable plasma. This consumes a larger volume of inert gas, argon gas for example, and thus reduces the life of the cryogenic pump which is used to pump out such gas. So, the on line availability of the sputtering apparatus decreases, because the pump must be replaced very often. If the pumping speed is decreased to save the injection gas, the contamination gas, especially gas released in the sputtering chamber, cannot be fully rejected. In this case, the quality of the sputtered film also decreases. This is a dilemma for the sputtering apparatus. In practical cases, the pumping speed is kept low to save the inert gas and to extend the pump life. Therefore, the quality of the sputtered grown thin film often suffers from contamination, and the production yield of high quality film because unstable.